Bug 805855 - Free dirty pages in response to all memory-pressure messages. r=jlebar

b2g/app/b2g.js

@@ -587,3 +587,7 @@ pref("general.useragent.override.facebook.com", "\(Mobile#(Android; Mobile");
 pref("general.useragent.override.youtube.com", "\(Mobile#(Android; Mobile");
 
 pref("jsloader.reuseGlobal", true);
+
+// Enable freeing dirty pages when minimizing memory; this reduces memory
+// consumption when applications are sent to the background.
+pref("memory.free_dirty_pages", true);

modules/libpref/src/init/all.js

@@ -3810,6 +3810,9 @@ pref("memory.low_memory_notification_interval_ms", 10000);
 // window to be collected via the GC/CC.
 pref("memory.ghost_window_timeout_seconds", 60);
 
+// Disable freeing dirty pages when minimizing memory.
+pref("memory.free_dirty_pages", false);
+
 pref("social.enabled", false);
 
 // Disable idle observer fuzz, because only privileged content can access idle

xpcom/base/AvailableMemoryTracker.cpp

@@ -5,22 +5,37 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/AvailableMemoryTracker.h"
-#include "nsThread.h"
-#include "nsIObserverService.h"
-#include "mozilla/Services.h"
-#include "mozilla/Preferences.h"
-#include "nsWindowsDllInterceptor.h"
+
 #include "prinrval.h"
 #include "pratom.h"
 #include "prenv.h"
+
 #include "nsIMemoryReporter.h"
+#include "nsIObserver.h"
+#include "nsIObserverService.h"
+#include "nsIRunnable.h"
+#include "nsISupports.h"
 #include "nsPrintfCString.h"
+#include "nsThread.h"
+
+#include "mozilla/Preferences.h"
+#include "mozilla/Services.h"
+
+#if defined(XP_WIN)
+#   include "nsWindowsDllInterceptor.h"
 #   include <windows.h>
+#endif
+
+#if defined(MOZ_MEMORY)
+#   include "jemalloc.h"
+#endif  // MOZ_MEMORY
 
 using namespace mozilla;
 
 namespace {
 
+#if defined(XP_WIN)
+
 // We don't want our diagnostic functions to call malloc, because that could
 // call VirtualAlloc, and we'd end up back in here!  So here are a few simple
 // debugging macros (modeled on jemalloc's), which hopefully won't allocate.
@@ -461,6 +476,86 @@ public:
 
 NS_IMPL_ISUPPORTS1(NumLowPhysicalMemoryEventsMemoryReporter, nsIMemoryReporter)
 
+#endif // defined(XP_WIN)
+
+/**
+ * This runnable is executed in response to a memory-pressure event; we spin
+ * the event-loop when receiving the memory-pressure event in the hope that
+ * other observers will synchronously free some memory that we'll be able to
+ * purge here.
+ */
+class nsJemallocFreeDirtyPagesRunnable MOZ_FINAL : public nsIRunnable
+{
+public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIRUNNABLE
+};
+
+NS_IMPL_ISUPPORTS1(nsJemallocFreeDirtyPagesRunnable, nsIRunnable)
+
+NS_IMETHODIMP
+nsJemallocFreeDirtyPagesRunnable::Run()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+
+#if defined(MOZ_JEMALLOC)
+  mallctl("arenas.purge", nullptr, 0, nullptr, 0);
+#elif defined(MOZ_MEMORY)
+  jemalloc_free_dirty_pages();
+#endif
+
+  return NS_OK;
+}
+
+/**
+ * The memory pressure watcher is used for listening to memory-pressure events
+ * and reacting upon them. We use one instance per process currently only for
+ * cleaning up dirty unused pages held by jemalloc.
+ */
+class nsMemoryPressureWatcher MOZ_FINAL : public nsIObserver
+{
+public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+
+  void Init();
+};
+
+NS_IMPL_ISUPPORTS1(nsMemoryPressureWatcher, nsIObserver)
+
+/**
+ * Initialize and subscribe to the memory-pressure events. We subscribe to the
+ * observer service in this method and not in the constructor because we need
+ * to hold a strong reference to 'this' before calling the observer service.
+ */
+void
+nsMemoryPressureWatcher::Init()
+{
+  nsCOMPtr<nsIObserverService> os = services::GetObserverService();
+
+  if (os) {
+    os->AddObserver(this, "memory-pressure", /* ownsWeak */ false);
+  }
+}
+
+/**
+ * Reacts to all types of memory-pressure events, launches a runnable to
+ * free dirty pages held by jemalloc.
+ * @see nsMemoryPressureWatcher::FreeDirtyPages
+ */
+NS_IMETHODIMP
+nsMemoryPressureWatcher::Observe(nsISupports *subject, const char *topic,
+                                 const PRUnichar *data)
+{
+  MOZ_ASSERT(!strcmp(topic, "memory-pressure"), "Unknown topic");
+
+  nsRefPtr<nsIRunnable> runnable = new nsJemallocFreeDirtyPagesRunnable();
+
+  NS_DispatchToMainThread(runnable);
+
+  return NS_OK;
+}
+
 } // anonymous namespace
 
 namespace mozilla {
@@ -468,7 +563,7 @@ namespace AvailableMemoryTracker {
 
 void Activate()
 {
-#if defined(_M_IX86)
+#if defined(_M_IX86) && defined(XP_WIN)
   MOZ_ASSERT(sInitialized);
   MOZ_ASSERT(!sHooksActive);
 
@@ -496,6 +591,12 @@ void Activate()
   }
   sHooksActive = true;
 #endif
+
+  if (Preferences::GetBool("memory.free_dirty_pages", false)) {
+    // This object is held alive by the observer service.
+    nsRefPtr<nsMemoryPressureWatcher> watcher = new nsMemoryPressureWatcher();
+    watcher->Init();
+  }
 }
 
 void Init()
@@ -509,7 +610,7 @@ void Init()
   // process, because we aren't going to run out of virtual memory, and the
   // system is likely to have a fair bit of physical memory.
 
-#if defined(_M_IX86)
+#if defined(_M_IX86) && defined(XP_WIN)
   // Don't register the hooks if we're a build instrumented for PGO: If we're
   // an instrumented build, the compiler adds function calls all over the place
   // which may call VirtualAlloc; this makes it hard to prevent

xpcom/base/AvailableMemoryTracker.h

@@ -10,10 +10,9 @@
 namespace mozilla {
 namespace AvailableMemoryTracker {
 
-// The AvailableMemoryTracker is implemented only on Windows.  But to make
-// callers' lives easier, we stub out empty calls for all its public functions.
-// So you can always initialize the AvailableMemoryTracker; it just might not
-// do anything.
+// The AvailableMemoryTracker launches a memory pressure watcher on all
+// platforms to react to low-memory situations and on Windows it implements
+// the full functionality used to monitor how much memory is available.
 //
 // Init() must be called before any other threads have started, because it
 // modifies the in-memory implementations of some DLL functions in
@@ -22,13 +21,8 @@ namespace AvailableMemoryTracker {
 // The hooks don't do anything until Activate() is called.  It's an error to
 // call Activate() without first calling Init().
 
-#if defined(XP_WIN)
 void Init();
 void Activate();
-#else
-void Init() {}
-void Activate() {}
-#endif
 
 } // namespace AvailableMemoryTracker
 } // namespace mozilla

xpcom/base/Makefile.in

@@ -43,6 +43,7 @@ CPPSRCS		= \
 		nsGZFileWriter.cpp \
 		nsMemoryInfoDumper.cpp \
 		nsMessageLoop.cpp \
+		AvailableMemoryTracker.cpp \
 		$(NULL)
 
 ifeq ($(OS_ARCH),Linux)
@@ -93,8 +94,6 @@ CSRCS		+= pure_api.c
 EXPORTS		+= pure.h
 endif
 
-CPPSRCS += AvailableMemoryTracker.cpp
-
 endif #if OS_ARCH == WINNT
 
 SDK_XPIDLSRCS   = \